Been trying phrase an answer to this for a few minutes and keep coming up with something that sounds like it's been written while half dunk on Christmas Day. which it has so with that disclaimer. structural proteins are ones where their function is related to 'structural roles'. things like actin or tubulin which form parts of the cells cytoskeleton, or collagen or fibronectin in the extra cellular matrix. the shapes off these monomers madness them suitable for building complex 3d structures. this is different from things like enzymes where their shape is critical too their ability to catalyse reactions. there are other biochemicals that can have structural roles such as large sugars things like hyaluronic acid but most structural elements are proteins, probably because of the diversity of different possible structures and the fact that cells already have systems for making, processing and transporting them.